Highly Efficient Organic Light-emitting Diodes Based On Improving Horizontal Dipole Orientation Of Emitters In Host-guest System

Organic light-emitting diodes(OLEDs)with the advantages of thinness,self-illumination and low energy consumption,have great application potential in flat display and solid-state lamp.With the rapid development of the OLEDs,the materials are constantly updated and the preparation technology is constantly upgraded.At present,phosphorescent materials with noble heavy metal atom and thermally activated delayed fluorescent materials with small energy gap between the lowest singlet(S₁)and triplet(T₁)states(?E_ST)have been widely used as emitters in OLEDs.These materials can effectively utilize triplet excitons to achieve luminescence,which can achieve 100%internal quantum efficiency in theory.In OLEDs,however,the difference of the refractive index between the organic material(n?1.7-2.0),glass substrate(n?1.5)and air(n?1)is too big.When the exciton in the light-emitting layer radiate light,most of the photons are limited in the device due to the base mode and waveguide mode,leading to a low device efficiency.Therefore,this thesis focuses on the method of improving the horizontal orientation of the light-emitting materials'transition dipole moment and the method is applied in the devices to improve the efficiency of OLEDs,preparing high-performance monochromatic and purely organic white OLED devices.In chapter 2,we propose a method to improve the horizontal orientation of light-emitting materials and verify the universality of this method.An orange light-emitting material TPA-AQ was used as the research object and its horizontal dipole ratio at different levels when it was doped in the host with the different horizontal dipole ratios which proved that the host with horizontally oriented transition dipole moments can effectively improve the horizontal dipole ratio of emitting material.The horizontal orientation of TPA-AQ in the doped film is more than90%with the host of DSpiroAC-TRZ and DSpiroS-TRZ.Such a high degree of horizontal orientation can avoid the waveguide mode and the base mode to some extent and improve the out-coupling efficiency of the device.The external quantum efficiency of OLED devices based on DSpiroAC-TRZ and DSpiroS-TRZ host are 17.8%and 20.5%respectively,which are about39%and 60%higher than those based on conventional CBP host.In chapter 3,the OLEDs based on thermally activated delayed fluorescent materials DSpiroS-TRZ with horizontal orientation had achieved high efficiency whatever in the doped or non-doped devices.Moreover,DSpiroS-TRZ can effectively improve the horizontal dipole ratio of TPA-AQ.Therefore,we combine these two materials with an appropriate device structure to build an all-fluorescent white OLED.Owing to the horizontally oriented transition dipole moments of DSpiroS-TRZ and TPA-AQ and the usage of triplet excitons,the maximum current efficiency of 72 cdA^-1 and the maximum external quantum efficiency of 28%was achieved in this all-fluorescent white OLED.Moreover,it shows a good spectrum stability.It's also the best performance among the reported all-fluorescent white OLEDs up to now.In conclusion,we propose a method to improve the horizontal orientation of the emitting material's transition dipole moment in OLED and achieve highly efficient monochromatic light and purely organic white OLED devices,which provides a reference for the structural design and performance optimization of highly efficient OLED devices in the future.